Moving laser beam readers or laser scanners, as well as solid-state imaging readers or imaging scanners, have both been installed in slot scanners or workstations having a horizontally and/or a generally vertically arranged window in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years, to electro-optically read targets, such as one-dimensional and two-dimensional symbols, particularly a Universal Product Code (UPC) bar code symbol, and non-symbol targets, such as driver's licenses, receipts, signatures, etc., the targets being associated with objects or products to be processed by the workstations. An operator or a customer may slide or swipe a product associated with, or bearing, a target in a moving direction across and past a window of the workstation in a swipe mode. Alternatively, the operator or the customer may momentarily present the target associated with, or borne by, the product to an approximate central region of the window in a presentation mode. The choice depends on user preference or on the layout of the workstation.
The known moving laser beam reader generally includes a laser scan engine or module for supporting an electrically energizable laser for emitting a laser beam through the window of the workstation, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances relative to the window, an electrically energizable scan component for repetitively scanning the beam spot across a symbol target in a scan pattern, for example, a scan line or a series of scan lines, across the symbol target multiple times per second, e.g., forty times per second, a photodetector for detecting return laser light reflected and/or scattered from the symbol target through the window of the workstation over a reading field of view, and for converting the detected laser light into an analog electrical signal, and electrical signal processing circuitry including a digitizer for digitizing the analog signal. Sometimes, the laser scan engine also supports a controller or microprocessor for controlling operation of the electrical components supported by the laser scan engine, and for decoding the digitized signal based upon a specific symbology used for the symbol target.
The known imaging reader includes an imaging scan engine or module for supporting a solid-state, image sensor comprising an array of pixels or light sensors, for sensing return light returning through the window of the workstation from a target being imaged. The image sensor may be a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and is analogous to the image sensors used in electronic digital cameras. The target can be a symbol or a non-symbol. The imaging scan engine also supports an illuminating light system for illuminating the target with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs), through the window of the workstation; an imaging lens assembly, e.g., one or more imaging lenses, for capturing return ambient and/or illumination light scattered and/or reflected from the target through the window of the workstation over a reading field of view and over a range of working distances; and electrical circuitry for producing electronic analog signals corresponding to the intensity of the light captured by the image sensor over the reading field of view, and for digitizing the analog signal. The imaging scan engine optionally supports a controller or microprocessor for controlling operation of the electrical components supported by the imaging scan engine, and for processing the target and/or decoding the digitized signal based upon a specific symbology when the target is a symbol.
Some workstations continuously capture and attempt to process and/or decode targets without regard to whether or not a target is actually in the reading field of view of the scan engine. However, continuous, repetitive, flashing of bright light from the LEDs of the illuminating light system in the case of the imaging scan engine, and continuous, repetitive, flashing of bright laser light from the laser in the case of the laser scan engine, consume and waste energy, degrade component lifetimes, and can be perceived as bothersome, distracting and annoying to the operators of the readers and to nearby consumers being served.
To alleviate these problems, the art has proposed periodically activating, i.e., waking-up, the scan engine with a few different approaches. For example, the scan engine can be operated with a very low duty cycle (about 10%). Thus, the scan engine wakes up for a very short period of time to scan the reading field of view and tries to detect a presence of a target therein. However, this creates sluggishness in the reader's performance and delays in decoding, which can be perceived as an engineering defect. Also, the distracting flashing laser light or illumination light is still present during the wake-up time period.
Another approach for the imaging engine tries to detect the target without energizing the illumination LEDs. However, under low ambient light conditions, for example, when a reader is located in a dark corner in a retail environment, the activation time will be slow since the exposure of the image sensor has to be very long in order to acquire an image of acceptable brightness for object detection. Again, the reader's performance is sluggish.
Still another approach is to install an object sensing system inside the workstation, for activating the scan engine, e.g., the illuminating light system, only if an object or product bearing, or associated with, a target is detected within the active reading field of view of the scan engine. The object sensing system has one or more object light sources for emitting object sensing light, typically infrared (IR) light, and at least one object sensor for sensing the return IR light reflected and/or scattered from the object over an object detection field of view. Although generally satisfactory for its intended purpose, the use of an internal object sensing system is disadvantageous, because a portion of the IR light incident on the window of the workstation is reflected therefrom back into the object detection field of view of the object sensor. This reflected portion of the IR light creates undesirable hot spots in the object detection field of view and may significantly compromise object sensor performance.
The art has proposed to eliminate hot spots caused not by the IR light, but by reflections of the illumination light off the window, by various means. For example, in the art of laser readers in which a laser beam is directed through a planar window to a symbol for reflection therefrom to a photodetector, it is known to tilt the planar window to prevent the laser beam incident on the window from reflecting back to the photodetector and compromising the detection and successful reading of the symbol. However, tilting the window, although acceptable in some applications, is not altogether desirable in other applications, for example, for an imaging reader, because the tilt angle required would be large, i.e., on the order of 45 degrees, which may be too large and difficult to implement in certain imaging readers and overly constrain the industrial design of the workstation. In the art of imaging readers, it is known to configure the window with spherical surfaces to prevent the illumination light incident on the window from reflecting back to the image sensor. This also constrains the industrial design of the workstation since, among other things, a spherical window is typically molded from plastic, and not glass.
It is desirable to have the active reading field of view of the scan engine relatively large at a near working distance or a close proximity to the window of the workstation so that the reading field of view covers the entire target. At farther working distances, it is preferred to have the reading field of view diverge slowly. A reading field of view with such characteristics is advantageously achieved by making the internal optical path between the scan engine and the window relatively long, and this is typically obtained by inserting a plurality of fold mirrors in this internal optical path to preserve a small compact volume for the workstation. However, such fold minors exacerbate the hot spot problem, because they constitute additional surfaces from which a portion of the IR light may be reflected therefrom back into the object detection field of view of the object sensor. All of these hot spots, also known as glare, are specular light, which can overload, saturate, and “blind” the object sensor, thereby additionally degrading object sensor performance.